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Evaluation of Alfalfa and Rhizobium meliloti for Compatibility in Nodulation and Nodule Effectiveness¹

The purpose of this investigation was to develop a procedure for testing strains of Rhizobium meliloti and cultivars of alfalfa (Medicago sativa L.) for symbiotic compatibility. Antiblotic-resistant mutants were developed from four strains of R. meliloti that differed for plant dry matter yield, days to initial nodulation and nodule numbers when inoculated onto one cultivar of alfalfa. The mutant strains were not significantly different from their corresponding parental strains in either days to initial nodulation or host dry matter production on N-free medium. The preference of alfalfa for different strains of Rhizobium was not altered by 0 to 300 ppm N and 33 to 300 ppm potassium phosphate.

The competitive ability for nodulation of effective and ineffective strains of R. meliloti was investigated using ‘Saranac’ and MnNC 4, an experimental population of alfalfa. Antibiotic-resistant mutants were used to verify the relative proportions of effective and ineffective nodules on each plant. A significant positive correlation was observed between the percentage of nodules produced by the effective strains and the dry matter production of plants grown at zero N. Acetylene reduction assays indicated that differences in competitive ability of the effective strains against the ineffective strains measured by the antibiotic-resistant mutant technique and seedling growth were associated with nitrogenase activity.

A procedure of inoculating alfalfa plants with a mixture of effective and ineffective strains at zero N and measuring shoot dry weight might be used to identify strains of Rhizobium of high competitive ability and plants with increased preference for effective strains of Rhizobium. This approach should expedite attempts to select Rhizobium-alfalfa combinations with enhanced N₂-fixation capacity.

Key Words: N₂ fixation • Inoculum • Antibiotic resistant mutant • Effectiveness assay • Competitiveness • Acetylene reduction

² Microbiologist, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota (present address: Joint FAO/IAEA Division, International Atomic Energy Agency, Seibersdorf Laboratory, A-2444 Seibersdorf, Austria); plant physiologists and research geneticist, USDA, SEA-AR, in the Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1509 Gortner Ave., St. Paul. MN 55108.

Received for publication September 25, 1980.